Roof bolts for use in mines, a method for their production and method for their installation

ABSTRACT

A roof bolt comprises a shaft for insertion in a hole drilled in a mine roof and a head whereby the bolt may be rotated, the head being offset with respect to the longitudinal axis of the shaft so that when the bolt is rotated by means of its head the shaft is caused to describe a circle of greater diameter than its own diameter, and where the offset is at least 0.08 inch and not more than 0.25 inch. Preferably the offset is from 0.10 to 0.16 inch. The invention includes a method for the production of roof bolts which comprise a shaft for insertion in a hole drilled in a mine roof and a head whereby the bolt may be rotated, the method being characterised by being controlled so that at least 90% of the bolts produced have their head central axis offset with respect to the longitudinal axis of the shaft by an amount of at least about 0.08 inch and not more than about 0.25 inch.

FIELD OF THE INVENTION

This invention relates to equipment for use in mines, more particularlyto roof bolts, to a method for their production and to a method fortheir installation.

BACKGROUND OF THE INVENTION

Roof bolts are well known in the mining industry and are extensivelyemployed for consolidating the roof and providing anchoring points andsupport. Their use involves drilling a hole in the roof of the mine andinserting into the hole a resin filled cartridge. The resin filledcartridges are also well known and comprise a curable resin and acatalyst for the resin contained in a sausage-like skin. In the nextstep of the installation a bolt is inserted which pierces the skin andpenetrates the resin. The bolt is then usually spun about its axis tomix the resin and catalyst and, once mixed, the resin cures and sets.

In some mines it is the normal practice to apply tension to the boltafter the resin has set. This may be done by providing a screw threadedportion on the bolt and screwing a-nut onto the threaded portion to abutan end plate until a certain predetermined torque is reached, which isnormally 120 to 150 ft lbs. This results in tension between the bolt andthe end plate.

PROBLEM TO BE SOLVED BY THE INVENTION

There is a long standing problem in coal mining particularly whenemploying small diameter bolts, of a phenomenon which is called glovefingering. The problem arises when the bolt is inserted into the hole.Insertion of the bolt causes pressure on the cartridge which forces theskin to the hole wall. The bolt then bores a hole through the contentsof the cartridge leaving the skin substantially intact. The result isincomplete direct contact by the resin with the wall of the hole andhence a less than optimum anchorage.

In mining operations in the USA there is widespread use of roof boltswhich are about ⅝ (five eighths) of an inch in diameter and these boltsare routinely employed in holes which are about 1 (one) inch indiameter. Another frequently used combination is a ⅞ (seven eighths)inch diameter bolt in a 1 (one) and ⅜ (three eighths) diameter hole.Glove fingering is a problem frequently encountered with thesecombinations of sizes of bolt and hole.

Previous attempts to solve the problem have involved the use of athinner skin to contain the resin, coarse filler in the cartridge andslash cut bolts. However these have generally achieved only limitedsuccess.

The present invention provides an alternative solution to the problem bycausing the bolt, when it is rotated or spun by means of its head, todescribe a circle of diameter greater then its own diameter.

One embodiment of the invention provides a novel form of roof bolt inwhich the head of the bolt is offset with respect to the axis of itsshaft by at least 0.08 inches.

SUMMARY OF THE INVENTION

According to the present invention there is provided a roof boltcomprising a shaft for insertion in a hole drilled in a mine roof and ahead whereby the bolt may be rotated and wherein the head is offset withrespect to the longitudinal axis of the shaft so that when the bolt isrotated by means of its head the bolt is caused to describe a circle ofgreater diameter than its own diameter, the amount of said offset beingfrom 0.08 to 0.25 inches.

The offset is preferably from about 0.10 to about 0.16 inches.

ADVANTAGES EFFECT OF THE INVENTION

The effect of the offset is that when a bolt is rotated by means of itshead, the shaft is caused to rotate about an axis which is offset withrespect to its longitudinal axis and describe a circle of diametergreater than its own diameter.

The result is to rupture the skin of the cartridge more effectively andthereby improve the contact of the resin with the wall of the hole. Inaddition mixing of the cartridge resin and catalyst contents is improvedresulting in an improved anchorage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) to FIG. 3(b) are longitudinal sections, not all drawn to thesame scale, showing the two halves of the gripper block die and theheader die at different stages of the production of the roof bolt.

FIGS. 4 is a schematic drawing which shows three steps in themeasurement of the offset.

FIGS. 5(a) and 5(b) show the installation of a roof bolt: FIG. 5(a) theconventional procedure where the axis of the roof bolt shaft is inalignment with the axis of the hole and FIG. 5(b) employing a novelchuck adapter which creates an offset between the two axes.

FIG. 6 is a side view of a bolt having an offset head and furtherincluding a wobble inducing member located on its shaft and

FIG. 7 is a side view of a bolt whose head is not offset including awobble inducing member located on its shaft.

DETAILED DESCRIPTION OF THE INVENTION

It has been observed that a small proportion of roof bolts which arecurrently available commercially have their head offset with respect totheir shaft. It is understood that this offset is accidental and arisesmerely on account of the engineering tolerances employed in theirmanufacture. Careful measurement of a large number of currentlyavailable roof bolts has established that the amount of offset is nevergreater than 0.07 inches. Bolts whose offset is 0.07 inches only exhibita marginal effect on the problem of glove fingering. Such bolts are notsuitable for use in the present invention with a conventional drillchuck although they may be used with the novel chuck adapter describedin the present specification.

The term roof is intended to embrace all surfaces of a mine such as walland floor as well as overhead surfaces.

The term mine is intended to include all earthworks requiring supportincluding quarries including tunnels.

References to the head being offset refer to the central axis of thehead. When the head is held in a conventional drill chuck and rotated inconventional manner, the head is rotated about its central axis. Thedegree of offset is measured by the method described in the presentspecification.

According to one embodiment the roof bolt includes a wobble inducingmeans such as a laterally projecting member attached to the shaft,preferably in the region of the end of the shaft distant from the head.

The laterally projecting member may be in the form of a stub and ispreferably located at, or close to, the end of the shaft of the bolt.

The laterally projecting member may be welded to the shaft or attachedby other means such as glueing. The laterally projecting member may alsobe integral with the shaft i.e formed in one piece with the shaft orformed from the shaft.

The rigidity and length of the laterally projecting member or stub issuch that on rotating the bolt by means of the head it will maintain thebolt tip in an offset position (ie push the end of the bolt to one sideof the hole) and thereby assist disintegration of the skin of thecapsule and result in a good bond between the resin and the wall of thehole. Typically the stub will project about 0.0625 to about 0.625 incheslaterally from the shaft. The length of the stub (the amount it projectslaterally from the shaft) is not more than the hole diameter minus thebolt diameter and will usually be about 50 to 80% of the differencebetween the hole diameter and the bolt diameter.

The stub is preferably located so that it is opposed, more preferablydiametrically opposed, to the offset in the head.

It has been found experimentally that the laterally projecting member orstub when used on a conventional roof bolt i.e. one not having an offsetcan effect a reduction in glove fingering.

Thus, according to another aspect of the present invention there isprovided a roof bolt comprising a shaft for insertion in a hole and ahead whereby the bolt may be rotated and where there is located at aposition on the shaft spaced from the head a laterally projecting memberhaving sufficient length and rigidity to cause wobble when the shaft isrotated.

The laterally projecting member may be provided by a stub as describedabove.

According to another aspect of the present invention there is provided amethod for the production of roof bolts said roof bolts comprising ashaft for insertion in a hole drilled in a mine roof and a head wherebythe bolt may be rotated, wherein the method is controlled so that atleast 90%, preferably at least 95%, of the bolts produced have theirhead offset with respect to the longitudinal axis of the shaft by anamount of at least about 0.08 inches and not more than about 0.25inches, preferably from about 0.10 to about 0.16 inches.

The method may comprise forming the bolts from lengths of steel rod byforging in a header machine said header machine comprising a gripper dieand a header die, the method further including accurately centering therod in the gripper die and forming the head by employing a header dieincorporating the offset in the construction of the header die.

The method preferably comprises centering the rod in the gripper die byheating the end to be headed to a length equivalent to the extendedlength calculated to fill the die plus the length of the gripper dieblock.

Conventional roof bolts, if they have any offset at all, always have anoffset of less than 0.07 inches and the present invention provides asystem that enables conventional roof bolts to be used in a manner thatreduces glove fingering. The system employs a novel chuck which isadapted to rotate the bolt so that the shaft is rotated about an axiswhich is offset from its own axis.

Thus according to a further aspect of the present invention a system forroof bolting comprises means for holding and rotating a roof bolt, saidmeans including a chuck for holding the head of the roof bolt andwherein the chuck is adapted to rotate the bolt so that its shaftrotates about an axis which is offset with respect to the axis of theshaft whereby the shaft is caused to describe a circle of diametergreater than its own diameter.

Preferably the chuck is adapted so that the offset is from about 0.08 toabout 0.25 inches.

According to another aspect of the invention a method for theinstallation of a roof bolt comprises:

-   -   inserting the roof bolt into a drilled hole containing a        cartridge comprising a curable resin and catalyst therefor so        that the bolt penetrates the curable resin and    -   rotating the bolt by means of its head to mix the resin and        catalyst to effect curing of the resin and where the axis of        rotation of the bolt is offset with respect to the longitudinal        axis of the shaft so that the shaft is caused to describe a        circle of diameter greater than its own diameter.

Roof bolt heads are conventionally formed by a process called upsetforging which is a process which increases the diameter of the end orcentral portion of a bar of metal by compressing its length. The boltsare made in a machine called an upsetter which consists of an electricmotor/flywheel arrangement which powers a slider-crank mechanism. Thelatter actuates a piston punch delivering a blow, and upsetting, the endof a slug.

Referring to FIGS. 1 a and 1 b: a slug 2 which is a precut length of rodstock is loaded, usually automatically, into an upsetter machineindicated generally by numeral 1. The slug 2 is gripped within a twopart die set 4 featuring a stationary die half 6 and a moveable die half8. With the die 4 in the open position (moveable half 8 slid out awayfrom the stationary half 6) the slug 2 is placed in the gripper blockdie cavity 7 of the stationary half 6 with a prescribed lengthsufficient to fill the cavity extending out from the die face (usuallyby means of a stop plate). The moveable half 8 slides inward grippingthe slug 2 as the punch 12 advances and imparts a particular plasticdeformation in, or upsets, the slug material 2, causing it to flow tofill the die cavity 14 (FIG. 1(b)). The die 4 then reopens while afinger mechanism (not shown) engages the finished part and ejects itfrom the die.

Upsetter forging machines used solely for the purpose of placing headson bolts are referred to as headers. Referring to FIG. 2: these are mostoften three stroke (cycle) with a 3 stage die set affixed to a headerdie and punch 20 and the slug 27 gripped between two gripping block diehalves 22 and 24. Such machines are used to form heads on rebar in themanufacture of roof bolts for the underground mining and tunnellingindustry. Although the punch/die tool alignment with the gripping blockdie cavity 25 is extremely precise, a slight variance exists in themanner in which the rebar stock 27 is gripped by the die blocks 22 and24, resulting in a slight-offset (typically less than {fraction(1/16)}^(th) (one sixteenth) inch between the rod or shaft axis and thatof the formed head. This is due in large part to the inability of thegripper die cavity 25 (designed to accommodate round bar stock ofuniform diameter) to evenly contact the non-uniform rolled deformations26 of the rebar 27. Since the maximum diameter of rebar stock 27 mayvary from 0.010 to 0.015 inches between any two axial points, a highprobability exists that the extent of effective grippable surface areaof the rebar 27 contacted by the gripper die at its front face differsfrom that at the back, thus allowing play 28 or room for swing at eitherend (see FIG. 2). Should the effective grip area be equivalent at thegripper block extremes, it is still probable that some points of contactbetween the internal surface of the gripper block die cavity 25 and therebar 27 at the front face of the gripper die block are diametricallyopposed to those at the back, allowing swing at either end. Hence uponcontact with the punch 20, an offset is produced between the shaft axisand that of the formed head. Again, the given magnitude of play in thegripper die results in an offset typically less than {fraction (1/16)}(one sixteenth) inch.

In a preferred embodiment of the present invention the rebar stock isaccurately centred in the gripping die prior to punch advancement andthe required degree of offset in the bolt is incorporated into thedesign of the header die.

As stated above the amount of offset should be from about 0.08 inch toabout 0.25 inch. Preferably the amount of offset is from about 0.10 inchto about 0.16 of an inch. One method of initially centering the stock 27in the gripping die 22, 24 is to heat the end 20 to be headed to alength equivalent to the extended length calculated to fill the headerdie plus the length of the gripper die block 22, 24. (FIG. 3 a). Thenusing a gripper block with a die cavity containing impressions of anapproved or matching rebar deformation pattern, the gripping pressurecan be adjusted to reform the gripped portion of heated rebar.

This enables the gripper blocks to simultaneously grip and form thestock, resulting in the elimination of play in the system (FIG. 3 b).The header die 20 can then incorporate the required degree of offset.

When the production is controlled in accordance with the presentinvention at least 90%, preferably at least 95% of the bolts have therequired degree of offset. The roof bolt conveniently includes a flangeto abut a plate of the type conventionally used with roof bolts. Such aplate is known in the art as a roof plate, and is conventionally used toabut the surface of the roof. The plate can be conveniently mounted onthe bolt and movable along the length of the bolt. The purpose of theplate is to spread the load and provide a firm seat for the head of thebolt.

In the installation of a roof bolt according to the invention a hole canbe drilled in the roof and a frangible cartridge containing a curableresin and catalyst inserted into the hole in a manner that is known inthe art. The bolt of the present invention can then be inserted into thehole and rotated to mix the resin and catalyst.

The invention is illustrated by the following Example.

EXAMPLE

Test bolts were made by modifying standard 4 feet by ⅝ inch diameterheaded bolts. The heads were cut off the bolts and a hole drilled inthem offset by 0.125 (⅛) inch from the center. One end of the bolts wasthreaded and the hole in the bolt head tapped to accept the bolt. Boltand head were tightly attached.

A four feet long resin cartridge was placed inside a four feet longsteel tube. The tube was sealed at one end representing a bore hole in amine roof. A modified bolt was inserted fully and spun for three secondsin accordance with established mine practice. Once the resin was hardthe tube was split and the contents examined for glove fingering. Theexperiment was repeated three times with the same type of modified bolt.The results are summarised in the Table. Test Number Glove fingering (%of grouted area) 1 0.9% 2 4.9% 3 6.9% 4  24%

Tests with the conventional i.e. non offset bolts of the same diametershowed glove fingering ranging from 55 to 100%.

% glove fingering is equal to the glove fingered area divided by thetotal surface area of the resin column.

In addition the resin and catalyst were mixed more effectively with theoffset bolts.

Measurement of Offset.

FIG. 4 illustrates an accurate method of measuring the offset. Themethod has been developed to eliminate error due to any angle that mayexist between the flange and shaft (i.e. the case where the shaft is notnormal to the flange). Such an angle would tend to lead to erroneouslyhigh values of offset.

Referring to FIG. 4:

(i) using a cut-off saw or band saw, the shaft is cut through just belowthe flange, so as to leave a small projection or stub of shaftapproximately 1{fraction (/16)}^(th) to ⅛^(th) inch high see 4(i). Thewidth, s of the bolt head is measured using a micrometer calliper.

(ii) the bolt head is inverted and mounted in the work piece vice of theCNC drill press machine see 4(ii). The drill is programmed toauto-adjust to locate a point at a distance of s/2 from the vice in boththe “x” and “y” directions. A pilot hole is drilled to a depth of{fraction (1/16)}^(th) inch or so to locate the center of the bolt head.

(iii) using a micrometer calliper, the center of the shaft projection islocated and marked. Similarly the distance between the head and shaftcenters is measured as the bolt offset see 4(iii).

Referring to FIGS. 5(a) and 5(b): a typical bolting cycle consists of(i) drilling a bore hole 40 (ii) lowering the drill boom to retract thedrill bit (or drill steel as it is commonly referred to in theindustry), (iii) placement of the roof bolt chuck adapter 42 in thebolter drill chuck (iv) manually inserting the cartridge(s) (not shown)in the hole (v) manual advancement of the bolt tip 44 of roof bolt 41into the hole 40 to retain the cartridge(s) (vi) placement of the headof the roof bolt 41 in the chuck adapter 42 (with roof plate (not shown)previously installed on the bolt) (vii) hydraulically raising the drillboom to fully insert the bolt 41 into the hole 40 and (viii)hydraulically actuating the bolter drill head to spin the bolt 41 to mixthe resin.

Upon lowering the boom (step ii), the bolter drill chuck and bore holeshould remain coaxial so that subsequent placement of the roof boltchuck adapter and roof bolt ensures that all components of the systemremain coaxial as well (refer to FIG. 5 a).

However where a ⅝^(th) inch bolt is inserted into a 1 inch hole, a ⅜inch total annulus results. This annulus is considered large for a 1inch hole and has been shown to significantly contribute to the problemof glove fingering, where the size of the annulus allows the bolt tobore through the central portion of the cartridge, leaving the cartridgefilm or skin intact between the resin contents of the cartridge and thehole wall. Subsequent spinning of the bolt is often ineffective inshredding the film as the large annulus prevents the generation of ashear stress between the bolt surface and the film sufficient to pullthe film away from the hole wall to be shredded.

In FIG. 5(b) there is shown a novel chuck adapter 45 which provides anoffset between the axes of the hole/drill and the bolt 44 such that theformer remains the axis of rotation, allowing a point on the outermostsurface of the bolt to generate a circle of greater diameter than thebolt itself (diameter=bolt diameter+2×offset). This effectivelydecreases the annulus by twice the offset, thereby increasing mixingshear and improving the shredding of the film. This offset is showndiagrammatically as a shift in axis in plane “Z”. This purely radialshift may be physically accomplished by cutting a typical adapter at “Z”perpendicular to its axis at some arbitrary distance from an end, andwelding the portions back together at an offset in the radial direction.However, the offset may be achieved by various machining andmanufacturing methods, utilizing a greater axial length of the adapter,so long as a resultant prescribed offset exists between the hole and thebolt axes.

Possible methods of manufacturing the adapter include forging, casting,multistage turning and cold working.

Referring to FIG. 6 which is a side view not to scale of a bolt whichcomprises a shaft 60 and flange 62 and a head 64 by means of which thebolt may be rotated. The central axis 65 of the head is offset withrespect to the axis 66 of the shaft and the bolt also includes at thetip of the shaft a stub 67 having sufficient length and rigidity toinduce wobble in the rotation of the bolt. The stub 67 is a shortprojecting piece of steel welded to the shaft 60. The stub 67 is about0.3125 inches in length and is attached at right angles to the shaft 60in the same plane and diametrically opposed to the offset in the head64. The length of the shaft of the bolt is 8 feet.

When the bolt is rotated by means of its head 64 the effect of the stub67 and the offset of the head is to cause a double cone wobble. In thecase of long roof bolts (typically greater than 5 feet in length andespecially greater than 6 feet or 8 feet) this is particularly effectivein causing disintegration of the skin of the capsule and thereby assistsbonding of the resin with the wall of the hole.

Referring to FIG. 7 a bolt 70 comprises a shaft 71 and flange 72 andhead 73 by means of which the bolt is rotated. The central axis 74 ofthe head 73 is in alignment with the axis of the shaft 71. Located onthe shaft 71 at a position remote from the head 73 is a laterallyprojecting member in the form of a stub 76 of steel welded to the shaftand having sufficient length and rigidity to induce wobble in therotation of the shaft.

1. A roof bolt comprising a shaft for insertion in a hole drilled in amine roof and a head whereby the bolt may be rotated and wherein thehead is offset with respect to the longitudinal axis of the shaft sothat when the bolt is rotated by means of its head the shaft is causedto describe a circle of greater diameter than its own diameter, andwhere said offset is at least about 0.08 inches and not more than about0.25 inches.
 2. A roof bolt as claimed in claim 1 wherein the offset isfrom about 0.10 to about 0.16 inches.
 3. A roof bolt as claimed in claim1 wherein there is located at a position on the shaft spaced from thehead a laterally projecting member having sufficient length and rigidityto cause wobble when the shaft is rotated.
 4. A roof bolt as claimed inclaim 3 wherein the laterally projecting member is located at a positionwhich is opposed, preferably diametrically opposed, to the offset in thehead.
 5. A roof bolt comprising a shaft for insertion in a hole and ahead whereby the bolt may be rotated and where there is located at aposition on the shaft spaced from the head a laterally projecting memberhaving sufficient length and rigidity to cause wobble when the shaft isrotated, wherein the length of the member is from 50 to 80% of thedifference between the hole diameter and the bolt diameter.
 6. A roofbolt as claimed in claim 5 wherein the laterally projecting member islocated at a position distant from the head which position is more thanhalfway along the length of the shaft
 7. A roof bolt as claimed in claim5 wherein the laterally projecting member is provided by a stub of steelwelded to the shaft or integral with the shaft.
 8. A method for theproduction of roof bolts said roof bolts comprising a shaft forinsertion in a hole drilled in a mine roof and a head whereby the boltmaybe rotated, the method being characterised by being controlled sothat at least 90% of the bolts produced have their head offset withrespect to the longitudinal axis of the shaft by an amount of at leastabout 0.08 inches and not more than about 0.25 inches.
 9. A method asclaimed in claim 8 which method comprises forming the bolts from lengthsof steel rod by forging in a header machine said header machinecomprising a gripper die and a header die, the method farther includingaccurately centering the rod in the gripper die and forming the head byemploying a header die incorporating the offset in the construction ofthe header die.
 10. A method as chimed in claim 9 which comprisescentering the rod in the gripper die by heating the end to be headed toa length equivalent to the extended length calculated to fill the headerdie plus the length of the gripper die block.
 11. A system for roofbolting, said system comprising a means for rotating a roof bolt saidmeans including a chuck for holding the head of the roof bolt andwherein the chuck is adapted to rotate the bolt so that the shaft of thebolt rotates about an axis which is offset with respect to the axis ofthe shaft whereby the shaft is caused to describe a circle of diametergreater than its own diameter.
 12. A system as claimed in claim 11wherein the chuck is adapted so that the offset is from about 0.08 toabout 0.25 inches.
 13. A method for the installation of a roof boltwhich method comprises inserting the roof bolt into a drilled holecontaining a cartridge comprising a curable resin and catalyst thereforso that the bolt penetrates the curable resin and rotating the bolt bymeans of its head to mix the teem and catalyst and where the axis ofrotation of the shaft of the bolt is offset with respect to thelongitudinal axis of the shaft whereby the shaft is caused to describe acircle of diameter greater than its own diameter and where the offset isfrom about 0.08 to about 0.25 inches.
 14. A method as claimed in claim13 which method comprises employing a chuck to rotate the head of thebolt, said chuck being adapted to rotate the bolt so that the shaft ofthe bolt rotates about an axis which is offset with respect to itslongitudinal axis so the shaft describes a circle of diameter greaterthan its own diameter.
 15. A method as claimed in claim 13 whichcomprises employing a roof bolt as claimed in claim 1 and rotating thebolt by means of its head to cause the shaft of the bolt to describe acircle of diameter greater than its own diameter.
 16. A roof bolt asclaimed in claim 2 wherein there is located at a position on the shaftspaced from the head a laterally projecting member having sufficientlength and rigidity to cause wobble when the shaft is rotated.
 17. Aroof bolt as claimed in claim 16 wherein the laterally projecting memberis located at a position which is opposed, preferably diametricallyopposed, to the offset in the head.
 18. A roof bolt as claimed in claim6 wherein the laterally projecting member is provided by a stub of steelwelded to the shaft or integral with the shaft.